The invention relates to a process for forming an edge seal for a phosphoric acid fuel cell and more particularly to a process for making an integral seal for such fuel cells using a hot compression process.
A typical phosphoric acid fuel cell stack is a series of individual cells separated by electrically conductive plates. Each cell comprises an anode layer, an electrolyte matrix layer and a cathode layer. The fuel and oxidant gases are directed to the anode and cathode through an array of parallel grooves in the electrically conductive plates. The fuel cell is normally rectangular or square in shape so manifolds can be positioned on the sides to supply and collect the process gases. Air-cooled stacks may have separate manifolds for directing cooling air through ducts in the some of the electrically conductive plates. Seals are employed at the edges of the electrodes to contain the process gases and to prevent intermixing at electrode edges and manifolds.
The cells comprise three components: an anode assembly, a carbon matrix, and a cathode assembly. The cathode assembly includes a porous graphite paper electrode support layer and the electrode layer. The anode assembly is similar with a non-electrically conductive layer applied thereto. Anode and cathode reactant gases are separated by the matrix layer and the non-electrically conductive layer in the cell interior region. Edge seals are required because the porous electrode support layer would allow reactant gases to escape at the perimeter of the electrode.
U.S. Pat. No. 4,756,981 describes forming an edge seal for a phosphoric acid fuel cell by increasing the density and reducing the porosity of the edge region by impregnating the edge region with inert powder having a particle size of less than one micron so that the capillarity resulting from the surface tension of the liquid electrolyte permeating the small pores forms an edge seal which will retain the gases within the cell.